⌛ Rust Async §

Concepts §

Futures §

= an operation that can be tested for completion

• std::future::Future
  • trait Future:
    • fn poll (cx) -> Poll<Self::Output>
      • cx: callback for when Poll changes
  • enum Poll<T>:
    • Ready(T) or Pending

Dealing with futures §

await §

• .await() -> polls the Future
  • if poll returns Ready(T), the expression will become the value of T
  • if pooll returns Pending, the call will return Pending to the own caller
  • when fn around an await call gets polled again, it will resume at the last await (like JS Generator)

async §

• async keyword before fn or a block {}
  • -> return type of fn will automatically be wrapped in a Future

Helper functions §

• task::block_on(future): polls future until it returns Ready
  • don’t use in async functions (blocks everything)
• task::spawn_local: adds a Future to a pool that can be polled by block_on
• task::spawn: adds the async taks to a threadpool, returns JoinHandle that can be awaited for a final value
  • Future to pass must implement send (🤹 Rust Concurrency)
• task::yield_now(): add a breakpoint to an async function
  • used to occasionally break from long async functions to avoid blocking
• task::spawn_blocking: moves async function to separate thread